The present invention relates to a UV detector which does not necessitate any electrical input. Specifically this UV detector is based on a photochromic material which changes color when exposed to UV radiation indicating in this way the presence of Ultraviolet radiation.
The sun which is fundamental for the existence of life form on planet Earth, emits in a large spectrum of wavelengths. This includes ultraviolet. Despite the undeniable importance of cutaneous exposure to ultraviolet radiation for vitamin D production, there is evidence that the skin is damaged in many different ways by its exposure to natural or artificial sunlight.
The ultraviolet spectrum is divided into three bands termed as UVA, UVB and UVC. The wavelength regions are defined by Environmental photobiologists as: UVA [320-400 nm], UVB [290-320 nm] and UVC [200-290 nm]. The division between UVB and UVC has been chosen to be at 290 nm since this limit represent the Ozone cutoff. Ultraviolet radiation that reaches the Earth's surface are UVA and UVB. UVA radiation is 1000-fold less effective than UVB in producing skin redness. However, its predominance in the total solar spectrum is 100-fold more than UVB. Therefore, UVA plays a more important role in contributing to harmful effects of sun exposure then previously suspected.
I. Atmospheric Ozone The amount of ultraviolet radiation reaching the Earth's surface depend on the energy output of the sun and the transmission proprieties of the Atmosphere. The quantity of UV radiation reaching the surface of our planet is largely governed by a thin layer of Ozone molecules (3 mm at standard temperature and Pressure). The Ozone O.sub.3 is created by dissociation of oxygen molecule O.sub.2 by short wavelength (.lambda.242 nm) in the region of the atmosphere called the Stratosphere. The recombination of the oxygen Atom and the molecular oxygen, in the presence of a catalyst molecule and absence of ultraviolet, is the phenomenon responsible of ozone formation. On the other hand, the dissociation of O.sub.3 by UV wavelength up to 320nm is the mechanism preventing radiation at wavelength less than about 290 nm from reaching the Earth's surface. This cycle of formation-dissociation of the ozone molecule is a very important process in maintaining life on Earth. In 1974, Molina and Rowland (Nobel prize of Chemistry 1995) first warned that chlorofluorocarbons CFCs and other gases released in the atmosphere by human activities could alter the natural balance of creative and destructive processes and lead to depletion of the stratospheric Ozone layer (Nature 249, 810). Reductions of up to 50% in the ozone column over Antarctica have been observed and first reported in 1985 by Farman and collaborators (Nature 315, 207). In addition, a significant decrease in total ozone over the northern hemisphere of about 2 to 3% per decade for the past 30 years has been reported by Frederick (Photochem. Photobiol. 51,757, 1990). Recent satellite measurements indicate a worldwide decrease in stratospheric ozone over the last decade.
II. Ultraviolet Intensity Variation
The spectral irradiance of UVR depends on the time of the day, the season, the geographical location, and multiple meteorological factors (Diffey, Phys. in Med. and Biol. 36(3), 299, 1991):
- Time of the day: 75% of the total UV radiation is received in mid-day, between 10AM and 3PM.
- Season: There is an undeniable seasonal dependence of UV radiation that reaches the earth surface. There is more Ultraviolet rays in summer than in winter. However, this seasonal variation is much less apparent near the equator.
- Geographical Latitude: Annual UVR flux decreases with increasing distance from the equator.
- Clouds: clouds have a minimal effect on intensity of ultraviolet radiation although they reduce the total solar irradiance at the earth's surface. However, very heavy storm clouds can eliminate terrestrial ultraviolet.
- Surface reflection: Reflection of UVR from ground surfaces is normally low although the reflectance of fresh snow exceeds 80%.
- Altitude: In general there is an increase of about 6% of UV flux each 1 km increase in Altitude.
III. Effects of Ultraviolet Radiation on Living Organisms The biological effects that result from ultraviolet radiation are initiated by photochemical absorption by biological molecules. The most important is the DNA deoxyribonucleic acid. The absorption spectrum of the DNA molecule, where the chromophores (absorbing centers) are the nucleotide bases, presents a maximum between 260-265 nm with a rapid decrease at longer wavelengths. This characteristic is useful as a way to get rid of unwanted viruses. Aquatic life is also affected by ultraviolet radiation. The organisms that live in either fresh water or oceans derive their energy from sunlight. Among them is the well known in the food chain Phytoplankton. Solar UV can penetrate to significant depths in water. Damaging effects of Solar UVB on phytoplankton will occur at depths in excess of 20 m in clear waters and 5 m in cloudy waters. It has been estimated that a reduction of 25% in the ozone layer would result in enhanced UVB levels at oceans that would lead to a decrease of 35% of phytoplankton photosynthesis (Smith et al, Photochem. Photobiol. 31,585, 1980). It is, also, believed that the increasing intensity of UV radiation could be responsible for the extinction of frogs. This can be explained by the sensitivity of their skin to their living environment.
The responses of plants to UV irradiation include the reduction of leaf size and limitation of the area available for energy capture. These results have been achieved through studies in greenhouses exposed to artificial sources of UV radiation (Tevini and Teramura Photochem. Photobiol. 50, 479, 1989).
IV. Effects of Ultraviolet Radiation on Humans
While a small amount of exposure to sunlight can be healthy, by inducing the synthesis of vitamin D, too much can be dangerous. Exposure to UV rays is linked to a number of harmful health effects.
1. Sunburn
Sunburn or erythema can be initiated upon excessive exposure to solar UVR. The resulting redness of the skin is due to an increased blood content in the superficial blood vessels in the dermis. Although it has been observed that UVA radiation is much less erythmogenic than UVB radiation (by a factor of 1000) the much higher UVA irradiance present in sunlight contributes about 20% to the sunburn (McKinley and Diffey 1987 in Human exposure to UV radiation: Risks and regulations, Elsevier, Amsterdam, pp83-87).
Of course, the skin color is an important factor in determining the ease with which the skin will sunburn. Fair-skinned people require only about 15-30 min. of midday summer sunshine to induce an erythemal reaction. People with moderately pigmented skin may require 1 to 2 hours exposure and those with darkly pigmented skin will not normally sunburn. Individuals can be grouped in six sun sensitive skin types as it is shown in the table below (Diffey, Phys. in Med. and Biol. 36(3), 299, 1991).
__________________________________________________________________________ Skin type Skin reactions to solar radiation Examples __________________________________________________________________________ 1 Always burns easily and People most often with fair skin, severely; tans little or none and blue eyes, freckles, unexposed peels skin is white 2 Usually burns easily and People most often with fair skin, severely; tans minimally or red or bond hair, blue, hazel or lightly, peels even brown eyes, unexposed skin is white 3 Burns moderately and tans about Normal average Caucasian, average unexposed skin is white 4 Burns minimally, tans easily and People with white or light brown above average, exhibits skin, dark brown hair, dark eyes, immediate pigment darkening unexposed skin is white or light (IPD) reaction brown 5 Rarely burns, tans easily and Brown-skinned persons substantially always exhibits IPD (Hispanics, East Indians, etc.), reaction unexposed skin is brown 6 Never burns exhibits IPD People with black skin (Black reaction African etc.), unexposed skin is black __________________________________________________________________________
2. Photo-aging
The signs of photo-aged skins are dryness, deep wrinkles, accentuated skin furrows, loss of elasticity and mottled pigmentation. It has been speculated that perhaps as much as 80% of solar-UV induced aging, occurs within the first 20 years of life. The relative importance of different wavelengths in aging can not be determined. However, some experiments have been performed on mice which showed that both UVA and UVB radiation are responsible in histological, physical, and visible changes characteristic of photo-aging. The application of sunscreens has been shown to inhibit photo-aging in mice chronically exposed to solar radiation. Maximum photo protection is afforded by chemical sunscreens with SPF ratings of 15 or higher. Although most sunscreens on the market today are appropriate for UVB protection, combination sunscreens that are effective against UVA and UVB are preferable.
3. Skin Cancer
Skin cancer is the most common human cancer. There is an undoubtable link between the occurrence of this cancer and the amount of exposure to UV radiation. It has been shown that Caucasians are much more likely to develop non-melanoma skin cancer NMSC. The action spectrum for the induction of melanoma by UVR is unknown, although UVB is considered by some scientists (Koh et al, Photochem. Photobiol. 51, 756, 1990) to be the waveband primarily responsible. The possibility that the action spectrum lies within UVA, visible or infrared cannot yet, be discarded.
4. Effects of solar ultraviolet radiation on the eye
Recent epidemiological studies have shown that UVB is the most damaging part of the ultraviolet spectrum for the eye. Studies in experimental animals have confirmed that the development of certain type of cataract (opacity of the lens) are associated with ocular UVB exposure. Similar results have been found in humans.
V. Monitoring Ultraviolet Radiation (Prior Art)
In order to better quantify and understand the UV problem, new monitoring programs are being put into place throughout the world. Because ultraviolet levels are dependent on local air pollution conditions, it is clear not only that many monitoring sites are needed but that the monitoring must be carried out continuously over a long period of time. A class of detectors called broadband instruments is used to extend UV monitoring to many sites and to all atmospheric conditions. The technology used in these sensors is based on fluorescent phosphors. The primary difficulty in measuring solar UV lies in the fact that less than 1% of the total energy of sun light is in this spectral region. The measurement background, in this case visible light, is many order of magnitude greater than the signal. Fluorescent phosphor MgWO.sub.4 (Magnesium Tungstate) based sensors utilize the combination of colored glass filters and the excitation-emission properties of the phosphor (D. S. Berger, Photochem. Photobiol. 24, 587, 1974 and D. Robertson, 1972, Ph.D. Thesis University of Queensland, Australia), See sales catalogue of Solar Light Co., Philadelphia, Pa. and Yankee Environmental Systems, Inc., Turners Falls, Mass. The instrument is composed of a UV transmitting weather dome, a black-glass UV pass filter, a fluorescent phosphor, and a photodiode. The UV radiation striking the Magnesium Tungstate phosphor is converted to visible light (green) which in turn is detected by a photodiode. The resulting photodiode current is proportional to the amount of UV light. A variety of phosphors and UV transmitters can be used to detect different spectral regions of ultraviolet light. Another sensor, used in meteorological applications to measure UVA, uses a Schott glass filter and BaMg.sub.2 Al.sub.16 O.sub.27 :Eu phosphor (B. K. Dichter, Journal of Machine Perception, v10 n4, 19, 1993).